Thermal mirage effect, also called heat haze or heat shimmer, refers to the tendency of objects to appear blurry or wavy when viewed through hot air. This phenomenon is caused by differences in refractive index between hot air and adjacent cooler air. Rays of light bend when they travel through a boundary between hot and cold air, which creates a distorted image. A rising plume of hot air is in constant turbulent motion, and the boundary between the hot air and surrounding cold air is continuously shifting. This causes the image of objects viewed through a plume of hot air to move and shimmer.
Thermal mirages can be extremely problematic for firearm shooters, especially when long distance target acquisition is desired. For example, long range target shooters rely on seeing a clear picture of their targets to aim their firearms accurately. A severe thermal mirage can cause a target to appear to move and shimmer, making it almost impossible to accurately hit the target. This problem is made worse by the fact that firearms themselves can become hot enough after multiple shots to produce thermal mirage-inducing plumes of hot air from firearm barrels or suppressors. Each time a firearm fires a bullet, the explosion that propels the bullet produces heat. After repeated firing, heat generated by passage of the bullets and hot gases convectively escapes outer surfaces of the firearm and heat surrounding air. This occurs especially quickly for firearms with muzzle attachments such as sound suppressors, energy capture systems, particulate capture systems, or visual signature reducers. These attachments tend to retain heat and convert acoustic and kinetic energy into additional heat. Then, hot air from the barrel or muzzle attachment rises and creates a thermal mirage in front of the sights or optical scope of the firearm, directly in a line of sight of the firearm operator.
Insulating covers for firearm suppressors have been developed. These covers insulate the suppressor from outside air, significantly slowing rates of heat transfer to air surrounding the suppressor. This can help to reduce thermal mirage effects because the suppressor does not heat the air quickly enough to create a mirage image in front of the sights of the firearm. However, because the insulating cover traps all the heat inside, the suppressor becomes even hotter than it normally would. The rest of the firearm will also begin to heat up more, until a thermal mirage effect is eventually created from hot air rising off the uninsulated barrel of the firearm and from the insulation. Permanent damage to the firearm can also result if sufficiently high temperatures are reached.
Heat sinks have also been developed for suppressors. These increase the rate of heat transfer from the suppressor to the surrounding air, causing the suppressor to cool more quickly. Heat sinks help prevent the firearm from overheating, and they may shorten the length of time that thermal mirage effects interfere with ballistic accuracy, but such devices do not prevent thermal mirage effects while the suppressor is hot. In fact, the thermal mirage effect may tend to be more severe because the heat sink transfers heat more quickly to the surrounding air within a given time period.